Spray-coating process



United States Patent Qfifice Bflififi'i'h Patented Feb. 26,- 19533,679,275 SPRAY-CGATlNG PRQCESS Michael G. Holowaiy, Gary, 3nd,,assign-or to inland Steel (loin-parry, Chicago, ill., a corporation ofDela" were No Drawing. Filed @ct. 12,1959, er. N 845,614- 4 Claims. (Cl.117-105) 'lhe presentinventionqrelates generally to a process of coatinga metal, surfacerwitha protective metal, and, more particularly to animproved process for continuously spray-coating a, ferrous metal basewith a thin, firmly adherent film of a. protective metal, sucha-saluminum, copper, tin, zinc, or'a protective alloy coating.

In a conventional metal spraying process in whichglobules of moltenmetal are projected onto a relatively cold base metal surface by acarrier gas, the metal globules solidify on striking the relativelycoldsurface of the base metal and are only mechanically bonded to thebase metal surface. It is, therefore, generally recognized that somemeans of heating either or both the coating and the base metal must beemployed in order to form a fusion bond between the coating and the basemetal. In, some instances, it has been proposed to heat thebase to anelevated temperature prior to, or simultaneously with, a coatingoperation, and inother in stances the coating is heated after beingdeposited in, the desired thickness on the base metal. When, however,the base metal is heated to an elevated temperature, it has beenobserved that an excessive amount of an intermetallic compound isfrequently formed, such as an aluminum-iron compound, tin-iron compound,or a zinc-iron compound, which results in a brittle and otherwise poorlyadherent coating. When the surface coating metal is maintained at arelatively low temperature during the coating operation to avoidformation of intermetallic compounds, there is often insufficientbonding between the coating and the base. There has thus been nocompletely satisfactory process devised for spray-coating a ferrousmetal base with a relatively thin, firmly adherent film of a protectivemetal of the foregoing class.

Accordingly, it is an object of the present invention to provide animproved metal spray-coating process which produces continuously, athin, strongly adherent spraymetal coating on a ferrous base.

It is a further object of the present invention to provide an improvedmetal spray coating which forms a secure fusion bond with a ferrousmetal base, while avoiding the formation of substantial amounts ofintermetaliic compound between the coating metal and the base metal.

It is a still further object of the present invention to produce animproved-process for continuously providing a thin, protective film ofaluminum which is firmly adherent to a ferrous metal base.

Other objects of the. present invention will be apparent from thedetailed description and claims to follow.

According to the present invention, one of the commonly employedprotective metals, suchas Al, Cu, Sn, Zn, or an alloy consisting of acombination of two or more of any of the said protective metals, isapplied to the surface of a ferrous metal base in a non-oxidizingatmosphere, and preferably in a neutral or a reducing atmosphere, byspraying dispersed droplets of the protective metal so thatsubstantially a single layer of droplets is sprayed and solidifies onthe ferrous metal base surface with each of the droplets being spacedfrom the next adjacent droplets so that a substantial portion of thesurface of the metal base may be exposed and not completely covered.Thereafter, and while still in a neutral or reducing atmosphere, thespray-coated surface is exposed rapidly and for a brief time, to atemperature above the melting point of the protective coating metal. Thedroplets are thus rapidly melted and a lateral flow of molten metaloccurs onto the exposed uncoated surface of the base metal which hasalso been heated rapidly and briefly as the droplets are melted, therebyrapidly forming a, continuous, thin film over the entire surface of thebase metal. During the brief heating, thereis a fusion of the coatingmetal which forms on the supporting strip a smooth, even surfacesubstantially non-porous and free of the usual surface defects which arefrequently encountered in metal coatings applied by spraying. Theatmosphere of the fusion chamber can be neutral or reducing, to avoidsurface oxidation, and specific compounds may be added to the atmosphereor can be deposited on the sprayed surface to promote formation of auniform, clean film of fused metal such as one of the various fiuxingagents in common use.

As soon as the protective metal has formed a smooth, even film orcoating on the base metal surface, as above described, and within amatter of a few seconds after exposure to the radiant heat, the metalstrip and coating are immediately and rapidly quenched to lower thetemperature of the base metal and protective coating, minimize theformation of the aluminum-iron and the like intermetallic compound, andto prevent the formation of objectionable amounts of the intermetalliccompound such as an aluminum-iron compound, which is brittle andfrequently results in poor adherence of the aluminum coatings. It ispreferred to use a liquid bath for rapidly quenching the base metal andcoating, such as water.

It is believed that the instant process provides an improved spraycoating because the discrete droplets of coating metal fuse more rapidlyon reheating than do a multiplicity of overlapping droplets whichcompletely cover the surface of the base metal, and because the metalbase is heated rapidly and does not remain at the elevated temperaturefor a prolonged period, since it is rapidly quenched immediately afterthe droplets flow onto the metal base. The instant process, however, isin no way to be limited by the foregoing postulated theory of operation.

in the application of the present invention to produce analuminum-coated strip, a continuously moving strip of steel (less than0.1 inch thick) is, after thoroughly cleaning to remove soil from thesurface thereof, continuously passed into a chamber containing anon-oxidiz ing atmosphere and beneath aluminum metal spray outletshaving the spray outlets and the rate of travel of the strip so adjustedthat discrete droplets of aluminum are deposited on the steel strip witha substantial portion of the steel strip between the. droplets remaininguncoated. it will be apparent that only in a continuous process of theforegoing type is it possible to get uniformly discrete droplets on thebase metal surface. The strip having the solidified spaced droplets ofaluminum thereon is then rapidly and briefly heated in a non-oxidizingatmosphere to a temperature of about 1200-1400 F. for a period of about2-10 seconds, by high density heat, such as produced by electric heatingelements or as produced in a luminous wall furnace wherein the gaspasses through the brick wall of the furnace and burns at the wallsurface, or by other radiant heat source. The aluminum droplets arerapidly fused by direct heat from the furnace wall and also byconvection heat from the metal strip which is also heated by directexposure to the heat, without heating the base metal completely,however, as in the hot dip aluminum process. The fused aluminum dropletsflow onto the heated metal surface between the droplet centers and forma smooth, continuous coating of aluminum having a thickness of aboutless than .0005-001 inch. Just as soon as the continuous coating hasbeen formed on the base metal strip, the strip is withdrawn from thefurnace and immediately and continuously quenched by. immersion in aliquid quenching bath, such as water, to rapidly cool the steel stripand the coating, thereby reducing to a minimum the forrnation ofintermetallic compounds normally resulting from the reaction between thealuminum and the iron of the base strip.

The same procedure is applicable for depositing a coating of otherprotective coating metals, such as copper, zinc, and the like, by makingsuitable adjustment in the furnace temperature depending on the meltingpoint of the protective coating metal employed. If desired, the steelstrip can also be pretreated with specific agents or compounds which areknown to inhibit formation of intermetallic compounds between the ironin the base strip'and the protective coating layer, such as molybdenum,but such pretreatment is not required as in the hot-dip aluminum coatingprocess.

It will be evident to one skilled in the art, that each of the steps inthe instant process is designed to avoid maintaining the ferrous basemetal and the protective metal coating while in contact therewith at anelevated temperature for any substantial or prolonged length of time inorder to minimize the heating of the base and the coating, and therebyreduce to an absolute minimum the formation of intermetallic compoundsat the interface between the base metal and the coating and preventingalloying to penetrate deeply into the surface of the base metal. This isof particular importance in the production of thin, protective coatingswhere a substantial proportion of the coating metal might be in the formof an undesirable intermetallic compound, such as the brittlealuminum-iron alloy compound, if positive steps were not taken to reducethe formation of an intermetallic compound. Thus, by spraying thecoating metal onto the base surface instead of dipping the base metalstrip into a molten bath of the coating metal, the entire base metalstrip is not heated to the temperature of the molten metal. Also, bysubjecting only the surface area of the spaced droplets of coating metaland the base metal to radiant heat, the droplets of coating metal arevery rapidly heated to the fusion temperature to form a continuouscoating before the base metal is heated to an elevated temperature. And,by immediately and rapidly quenching the base metal and the surfacecoating metal as soon as a uniform coating is formed, a minimum ofintermetallie compound is formed. It will also be apparent that theinstant process in one preferred form is particularly suitable forapplying a thin, protective metal coating to a thin strip or sheet of aferrous metal, since a thin sheet can be more rapidly cooled in thefinal quenching step.

Others may practice the invention in any of the numerous ways which aresuggested to one skilled in the art, by this disclosure, and all suchpractice of invention are considered to be a part hereof which fallwithin the scope of the appended claims.

I claim:

1. A continuous process of spray-coating a ferrous metal base with athin protective metal which comprises: continuously spraying a moltenprotective metal on a ferrous metal base surface with the protectivemetal forming discrete spaced droplets on the surface of said metal baseand leaving exposed a portion of the said base; and rapidly quenchingsaid heated metal base and protective metal coating to prevent formationof objectionable amounts of intermetallic compound therebetween.

2. A process as in claim 1, wherein the protective metal 7 is selectedfrom the group of metals consisting of aluminum, copper, tin, zinc, andan alloy combination of the foregoing metals.

3. A continuous process of spray-coating a ferrous metal base with athin protective metal which comprises: continuously spraying moltenaluminum on a ferrous metal base surface, said aluminum being sprayed onthe base surface as discrete spaced droplets while leaving a portion ofsaid metal surface exposed between the spaced droplets of aluminum, saidspraying operation being carried out in a nomoxidizing atmosphere;exposing briefly said aluminum spray-coated metal surface while in anonoxidizing atmosphere to a source of radiant heat to rapidly melt thealuminum droplets which have solidified on the base surface and permitsaid droplets to how together forming an uninterrupted aluminum coatingon the base surface; and rapidly quenching said heated metal base andaluminum coating to reduce formation of an intermetallic aluminum-ironcompound therebetween.

4. A continuous process of spray-coating a ferrous metal base strip witha thin protective coating of aluminum which comprises: continuouslyspraying molten aluminum on a continuously moving strip of a ferrousmetal, said aluminum being deposited on said strip in discrete spaceddroplets with the surface of said strip between the spaced dropletsbeing uncoated with aluminum and exposed to the surrounding atmosphere,said spraying being carried out in a non-oxidizing atmosphere;continuously exposing said aluminum spray-coating and said metal stripbriefly to a source of radiant heat to rapidly melt the aluminum andheat said strip, whereby the droplets flow together forming anuninterrupted aluminum coating on said strip; and thereafter immediatelyand rapidly quenching said strip and aluminum coating in a liquid bathto prevent formation of objectionable amounts of an intermetallicaluminum-iron compound therebetween.

References Cited in the file of this patent UNITED STATES PATENTS2,166,510 Whitfield et al July 18, 1939 2,361,962 Ronay Nov. 7, 19442,414,923 Batcheller Jan. 28, 1947 2,721,156 Steuck Oct. 18, 19552,839,437 Manko June 17, 1958. 2,845,366 Schroeder July 29, 19582,864,696 Foreman Dec. 16, 1958 2,926,103 Brick Feb. 23, 1960 2,955,958Brown Oct. 11, 1960 2,965,963 Batz et al Dec. 27, 1960 2,982,016Drummond May 2, 1961 3,012,904 Baer et a1 Dec. 12, 1961

1. A CONTINOUS PROCESS OF SPRAY-COATING A FERROUS METAL BASE WITH A THINPROTECTIVE METAL WHICH COMPRISES: CONTINUOUSLY SPRAYING A MOLTINGPROTECTIVE METAL ON A FERROUS METAL BASE SURFACE WITH THE PROTECTIVEMETAL FORMED DISCRETE SPACED DROPLETS ON THE SURFACE OF SAID METAL BASEAND LEAVING EXPOSED A PORTION OF THE SAID METAL SURFACE BETWEEN THEDROPLETS, SAID SPRAYING OPERATION BEING CARRIED OUT IN A NON-OXIDIZINGATMOSPHERE; EXPOSING SAID SPRAY-COATED METAL SURFACE WHILE IN ANONOXIDIZING ATMOSPHERE TO A SOURCE OF HEAT SUFFICIENT TO RAPIDLY MELTSAID SPACED DROPLETS AND HEAT THE EXPOSED METAL SURFACE, THEREBYPERMITTING SAID DROPLETS TO FLOW TOGETHER AND FORMING A PROTECTIVECOATING OVER SAID METAL BASE; AND RAPIDLY QUENCHING SAID HEATED METALBASE AND PROTECTIVE METAL COATING TO PREVENT FORMATION OF OBJECTIONABLEAMOUNTS OF INTERMETALLIC COMPOUND THEREBETWEEN.